In recent years, along with the popularization of personal computers into homes, digital still cameras (hereinafter referred to simply as digital cameras) that enable input of picture image information, such as photographed landscapes and portraits, into a personal computer are rapidly becoming more popular. Additionally, with enhancements in portable telephone functions, portable cameras that include compact imaging modules are rapidly becoming more popular. Additionally, including an imaging module in compact information terminal equipment, such as PDAs (Personal Digital Assistants), is becoming popular.
In such devices that include an imaging function, an image pickup element, such as a CCD (Charge Coupled Device) or a CMOS (Complementary Metal Oxide Semiconductor), is used to provide the imaging function. Recently, advancements in the miniaturization of such image pickup elements have been rapidly increasing. This has resulted in a desire for the main body of such devices and other imaging equipment, such as the imaging lens system, to also be further miniaturized. Additionally, image pickup elements with a larger number of pixels in the same area have been developed in order to achieve higher image quality, which creates a demand for higher resolution lens systems that are still very compact. Japanese Laid-Open Patent Application 2000-258684 describes exemplary single focus imaging lenses for such devices that include only two lens elements.
As stated above, recent image pickup elements are smaller and provide more pixels in a given detector area, which helps meet demands of higher resolution and more compactness that are especially required in imaging lenses for digital cameras. Although considerations of small cost and compactness have been the main considerations for imaging lenses for compact information terminal equipment, such as portable telephones with cameras, such devices have been commercialized with megapixel detectors (detectors that detect one million or more pixels), indicating increasing demand for higher performance in these devices as well. Therefore, development of lens systems with a wide range of applications based on properly balancing considerations of cost, performance, and compactness is desired.
As imaging lenses for compact information terminal equipment having a large number of pixels, conventionally there has been developed a lens system having three lens components, each of which may be a lens element, with at least two lens elements being made of plastic, while the third lens element may be made of plastic or glass. However, in order to meet recent demands for greater miniaturization, a lens that uses a smaller number of lens components and lens elements, but which is equivalent in performance to these conventional lenses, is desired. Although the lenses described in Japanese Laid-Open Patent Application 2000-258684, referenced above, each have a two-component, two-element lens construction, which includes aspheric surfaces, a lens system that is even more compact and higher in performance is desired.
In order to achieve high performance in imaging lenses generally, ideally all types of aberrations should be favorably corrected over the entire region of the field of view. In general, in an optical system having a large number of lens components and lens elements, nearly ideal performance may be easily achieved. However, as the number of lens components and lens elements is reduced, it becomes more difficult to achieve nearly ideal performance. As noted above, in recent years, a great demand for a high performance imaging lens having only a small number of lens components and lens elements has developed.
Therefore, conventionally, optical designs have been developed to achieve, as far as possible, excellent correction of various aberrations over the entire field of view even when the number of lens components and lens elements is small. However, with a very small number of lens components and/or lens elements, such as only two or three, it is difficult to adequately correct all of the aberrations over the entire field of view. Even so, even if aberrations partially remain, when a person views an actual imaged picture, as long as the adverse effects to the actual imaged picture viewed by an observer are small, the problems of the aberrations are considered to be practically solved.
For example, in many cases a conventional imaging lens is often designed such that optical distortion aberration of the imaging lens is minimized over the field of view. However, a picture viewed on a monitor screen is also subject to TV distortion independent of the optical distortion of the imaging lens, and therefore TV distortion should also be considered in designing an imaging lens in order to minimize the distortion of a picture when viewed on a monitor screen. For example, although optical distortion of the imaging lens may occur over the field of view, the perceived distortion of a viewed picture on a monitor screen may become unnoticeable by properly balancing optical distortion of the imaging lens and TV distortion.
Concepts of optical distortion of an imaging lens and TV distortion will now be described with reference to FIG. 8. FIG. 8 schematically shows a rectangular object that is imaged via an optical system and displayed on a TV screen. In FIG. 8, a broken line 10 shows the rectangular shape that would be the ideal representation of the rectangular object, and a solid line 11 shows the shape of an actual image that is displayed.
With reference to FIG. 8, if the ideal image height is y0 and the actual image height is y, the amount of aberration of optical distortion D is generally expressed by the following equation:D=[(y−y0)/y0]×100 (%). Namely, the optical distortion D is defined by dividing the difference between the actual image height y and the ideal image height y0 by the ideal image height y0 and multiplying the quotient obtained by 100 percent in order to express the optical distortion in percentage terms.
On the other hand, again with reference to FIG. 8, the TV distortion Dt is defined by dividing the depth of curvature Δh of the long side of the actual image that ideally would have no curvature by twice the vertical height h (i.e., as measured from the optical axis, which corresponds to the center of the T.V. image) of a shorter side of the actual image and multiplying the quotient obtained by 100 percent in order to express the optical distortion in percentage terms. Thus, the TV distortion is defined by the following equation:Dt=(Δh/2h)×100(%). 